Agricultural Drainage

In Hooghoudt's equation, if the impermeable layer is located at a very large depth below the drain level, the flow becomes radial near the drain and the equivalent depth $d$ approaches a limiting value. For a drain spacing $S = 50\text{ m}$, drain pipe radius $r = 0.10\text{ m}$, and depth to barrier $D = 8.0\text{ m}$, Hooghoudt's equivalent depth is $d = \frac{D}{1 + \frac{8 D}{\pi S} \ln(D/r_0)}$. Taking $\ln(8.0/0.10) = \ln(80) = 4.382$, the equivalent depth $d$ in meters is ________ (round off to 2 decimal places).